Electric box - extension for clamp?

[Randy Gazda]

I will try my best to explain the problem. I am installing exterior metal boxes (round) for outside lights. I planned to put a clamp on the back of the box where the wires enter and exit. However, after going through 1/2" osb and 3/8" siding, there is no way I can get to a normal clamp to tighten it. Do they make an extension for the clamps, like a 1/2" pipe nipple? Whatever I do has to pass inspection.

Thanks in advance for the advice!

[Lewis Anderson]

Assuming you are using a regular weathertight box with threaded holes...

Can you not take the box off the wall, put the clamp and wire into the box, tighten it, and then remount the box?

If not you can use regular 1/2" galvanized pipe coupling and close nipple to extend it.

[Randy Gazda]

Lewis, I could do that if I drilled a much bigger hole in the wall, maybe. I put a 3/4" hole in the wall, pulled wire and installed box, I now realize I need a clamp and also wonder how I keep it the the standard 1.5" from the wall on inside. It seems like they should make an extension but maybe that is conduit?

[Lewis Anderson]

I don't know of any electrical fitting that will fit through a 3/4" hole. A 1/2" conduit nipple is 7/8" od and conduit is the only sort of extension that is made. So I think either way you do it you will need to drill a bigger hole, 1 1/8" or so.

[Charles Lent]

When doing this it is common to use a pipe nipple to go through the wall to another box mounted inside. Size the length of the nipple based on the wall thickness plus 1" and the diameter based on the code tables for wire fill of rigid conduit. A galvanized pipe nipple is the same diameter as rigid conduit. You should thread this nipple into the rear of the outside box and then mount the outside box to the wall. You would then use drive nuts to secure the inside box to the nipple and then mount this box to the inside wall. This inside box could then be the junction point of the conduit for your inside wiring.

Charley

[Buck Williams]

Maybe I'm not picturing this correctly You have 7/8" of siding and sheathing with a 3/4" hole drilled in it for romex. Just guessing but a 2 screw clamp style romex clamp amp may be 1-1/8 - 1-1/4 in total outside diameter, but the overall length of the clamp once the threads are screwed into the box is less than 1". You should be able to either drill out or hack out (with a chisel) enough depth in the siding and sheathing to allow you to screw the clamp into the box, and set the box back into the recess that you made for the romex clamp. Doesn't have to be pretty, the box is going to cover it up, no one will know if you had the right size forstner bit, or if you beat the crap out of it with a chisel.

[Randy Gazda]

Buck, I could do that, however, this is a commercially produced product. There must be a correct and neat way to install (I have 5 of them to complete).

Flex conduit?


Maybe I'm not picturing this correctly You have 7/8" of siding and sheathing with a 3/4" hole drilled in it for romex. Just guessing but a 2 screw clamp style romex clamp amp may be 1-1/8 - 1-1/4 in total outside diameter, but the overall length of the clamp once the threads are screwed into the box is less than 1". You should be able to either drill out or hack out (with a chisel) enough depth in the siding and sheathing to allow you to screw the clamp into the box, and set the box back into the recess that you made for the romex clamp. Doesn't have to be pretty, the box is going to cover it up, no one will know if you had the right size forstner bit, or if you beat the crap out of it with a chisel.

[Mark Major]

Randy, they make plastic inserts that are self "clamping" for romex. They just snap into the box knock-out and a tongue of sorts inside the insert allows the wire to only go one way essentially acting like a clamp. They're code complient here in Canada and I would imagine in the U.S. as well. You'd still have to pull the box and drill a larger hole in the siding/sheeting to accept the insert though. HTH

Mark.

[Tom M King]

Can you staple it close to the box inside the wall?

[Randy Gazda]

Mark, I tried one I had but it is 1/2" and will not fit the 1/2" threads (made for knockout). Do you know if they make a 3/8"?

Randy, they make plastic inserts that are self "clamping" for romex. They just snap into the box knock-out and a tongue of sorts inside the insert allows the wire to only go one way essentially acting like a clamp. They're code complient here in Canada and I would imagine in the U.S. as well. You'd still have to pull the box and drill a larger hole in the siding/sheeting to accept the insert though. HTH

Mark.

[Randy Gazda]

Tom, the locations are within 8" , but I can't keep it 1.5" from the wall (I think that is required by code?)?

Can you staple it close to the box inside the wall?